In a low noise amplifier for use in a power amplifier for amplifying linearly modulated signals or a receiver for linearly modulated signals, high linearity is required to suppress the deterioration of a spectrum characteristic and a transmission characteristic caused by a signal distortion.
In particular, when a multilevel amplitude modulation system is applied to radio communication, by linearizing the amplitude characteristic of the power amplifier, a technique is required to suppress nonlinear distortion and reduce adjacent channel leakage power. Also, in general, although high power efficiency is always required for the amplifier, the linearity and the efficiency of the amplifier are generally contradictory characteristics, and when the improvement of power efficiency is intended using an amplifier having degraded linearity, a technique for compensating distortion produced thereby is inevitably needed.
As one of such the distortion compensation methods, a predistortion system is known. The principle of the predistortion system is to obtain a desired signal without distortion in the output of an amplifier by adding in advance an inverse characteristic of the distortion characteristic of the amplifier to an amplifier input signal.
Typically, in distortion compensation processing according to the predistortion system, a transmission signal before distortion compensation (reference signal) is compared with a feedback signal output from a power amplifier, and by use of the error thereof (power difference), a distortion compensation coefficient is calculated and updated. The distortion compensation coefficient is stored in a memory, of which address is determined from amplitude or power of the reference signal or the function thereof. Then, by multiplying the updated distortion compensation coefficient by a reference signal for subsequent transmission, an adjustment is made such that a reference signal gain comes to have an inverse characteristic of the distortion characteristic of the power amplifier. Then, the above gain-adjusted transmission signal is input to the power amplifier. By the repetition of the above operations, a convergence is made finally to an optimal distortion compensation coefficient, and thus, the distortion of the power amplifier is compensated.
In the above distortion compensation processing, through the measurement of the delay time in the feedback signal, it is possible to obtain an accurate distortion compensation coefficient by making a transmission signal time coincide with the feedback signal time.
The delay time in the feedback signal includes a delay in the power amplifier and a delay in an analog circuit on a feedback signal path. The above delay time is varied by various factors such as an external temperature and a secular change.
As a first method for adjusting the delay time variation in the above feedback signal, there is known a method for adjusting the delay by performing fast Fourier transform (FFT) on the feedback signal and by using the above FFT calculation result (Patent document 1). More specifically, using the FFT calculation result, the distortion compensation apparatus calculates one of the values of a signal-to-noise ratio, an adjacent channel leakage power ratio, and a noise level. By the comparison of the calculated value of the present time with the calculated value of one time before, the feedback signal delay is adjusted.
Also, as a second method, by the extraction of a certain range of time series of reference signals before distortion compensation and feedback signals, self-correlation values of both signals are obtained while each time of the above signals is relatively varied. Then, a time difference forming a maximum self-correlation value is determined to be a feedback signal delay time.
However, according to the above first method, there is applied feedback control to adjust the delay time to make the difference in the calculated values zero, instead of quantitatively obtaining the exact feedback signal delay time. Therefore, it takes a substantial time before an optimal delay time is obtained.
Also, according to the above second method, because the self-correlation property of the reference signals is relatively weak, and a definite peak value is not obtainable in the vicinity of the maximum value of the self-correlation values. This produces an error in the maximum value of the self-correlation values, and an accurate delay time is not obtainable. To cope with the above problem, the self-correlation calculations are executed for a plurality of number of times, so that the self-correlation values are accumulated. Thus, processing to eliminate the error from the self-correlation value to the possible extent is performed. However, this brings about the problem of producing an increased processing time and a low tracking speed.    [Patent document 1] International Publication No. WO 2002/087097.